Petroleum refinery equipment components experience varying degrees of high temperature erosion and corrosion. Typical components affected include, for examples, piping elbows, nozzles, valve seats and guides, thermowells, and pump internals. Ethylene is produced by cracking petroleum feedstocks, such as ethane and naphtha, at temperatures up to 1150° C. (2100° F.), thus making the process gas stream inside the tube highly carburization. The furnace tubes suffer both carburization and coking on the internal surface of the tube. In order to maintain the process efficiency, the coke deposits have to be regularly removed from the tube inner diameter (ID) surface by a process referred to as “decoking” at approximately 300° C. (550° F.), which involves injecting a mixture of steam and air into the furnace tube. Thus, the high temperature and wear resistant alloy components of radiant furnace coils are often observed to suffer from severe erosion damages caused by impingement of coke particles generated during ethylene cracking process. The most erosive experience is during the internal tube cleaning process called “spalling” at about 700° C. (1300° F.).
Short-term solutions include modifying various process parameters to reduce the extent of coke deposition or increasing the frequency of decoking to minimize wear. Some fitting designs include a heavy outside wall to absorb erosion from coke particles and during decoking. For example, for coker heaters, the last four return bends in the radiant section may have heavier wall thicknesses. However, these designs can suffer thermal fatigue as a result of the cyclic nature of regular operations, decoking, and startup and shutdowns. That is, generally speaking, thicker, non-uniform walled tubes and other components are more prone to thermal fatigue, so this solution has been imperfect.
Longer-term solutions are to apply wear and corrosion protective coatings to the components. However, hard-facing an inner surface of a pipe has proved very difficult because the line of sight is lost. Such weld deposits are also subject to overlay cracking, underbead cracking and cracking into the base material.
Boron, carbon, and nitrogen diffusion coatings have also been promoted to retard coke build up. However, fabrication issues have prevented the coatings from having much success in industry.
High temperature abrasion, erosion and corrosion resistant components in refineries have been in some instances manufactured from Co—Cr—W alloys incorporating a generous amount of Cr and W. They have been castings of these alloys in some instances, and deposition of wear-resistant Co—Cr—W alloys by hard-facing onto steel substrates in other instances. Wrought Co—Cr—W alloys have also been used. These solutions to this long-standing problem has been satisfactory, however, because castings of these alloys are especially expensive and difficult to make, and hard-facing suffers from line-of-sight, heat-affected zone, and other problems.
A number of prior patents illustrate the state of the art in this technical field of imparting wear and abrasion resistance to pipe interiors. For example, U.S. Pat. No. 4,389,439 to Clark et al. discloses an erosion resistant diffusion coating on the surface having an inner layer comprising intimately dispersed iron carbide and an outer layer consisting essentially of iron boride for the tubular apparatus for handling slurries.
U.S. Pat. No. 4,641,864 to Heine et al. discloses an abrasion resistant pipe bend or elbow for slurry pipelines. The bend or elbow has a wall of enlarged thickness includes a plurality of spaced protrusions. Leading edges of the protrusions optionally have a cladding of an abrasion resistant hardfacing composition disposed for example by laser cladding.
U.S. Pat. No. 5,873,951 and No. 6,537,388 to Wynns et al. disclose diffusion coated ethylene furnace tubes. The inner surface of the ethylene furnace tubes is diffusion coated with a sufficient amount of Cr or Cr and Si to form a first coating having a thickness of at least two mils. A second coating of a sufficient amount of Al or Al and Si is diffused onto the first coating to form a total coating thickness of at least five mils.
U.S. Pat. No. 6,187,147 to Doerksen discloses return bend elbow fittings in a delayed coker furnace which are improved by subjecting the inner surface of the fittings to a boron diffusion hardfacing process and forming a hardened layer typically a few thousandths of an inch in thickness.
U.S. Pat. No. 6,413,582 to Dong-Sil Park et al. discloses a method for slurry coating internal surface of a superalloy substrate. The slurry contains a variety of aluminum-containing materials such as aluminum, platinum aluminide, nickel aluminide, platinum-nickel aluminide, refractory-doped aluminides, or alloys which contain one or more of those compounds. The coating is diffusion bonded to the substrate at temperatures from 1800 F to 2100 F. The coating thickness varies from 0.005″ to 0.010″.
U.S. Pat. No. 6,749,894 to Chinnia G. Subramanian et al. discloses corrosion-resistant thin coatings (0.004-0.400″) for steel tubes. The coating methods are PTAW, CVD, thermal spray and also slurry coating followed by reactive sintering at a temperature in the range of 1112 F to 2192 F, preferably in the range of 1742 F to 2102 F. The powders used are crushed and 2 to 10 μm and 50 to 150 μm powders are blended together. Carbon content has to be very low in order to maintain good corrosion resistance. Typical alloy examples are UNS N10276 and UNS N06200. Also silicon is included in the blended powders to lower the melting point during reactive sintering. Some or all of the powder preferably has an angular, irregular or spikey shape. The coating material contains up to 1.0 wt % Y, Zr, Ce and C.
U.S. Pat. No. 7,615,144 to Devakottai et al. discloses a thermal cracking process that employs at least one bend fitting carrying a protective layer comprising a steel carrier and carbide pellets applied by MIG welding or plasma arc welding.
Accordingly, the industry has remained in need of a solution to high temperature erosion and corrosion of pipe interiors, especially at returns and bends.